1. Field of the Invention
This invention relates to an electrical heating device and a method for the manufacture of a device of this nature.
2. Description of the Related Art
A generic electrical heating device has an open housing, in which a layer structure with at least one radiator element, at least one heat emitting element and at least one PTC element, as well as at least one spring element are accommodated. The spring element is accommodated in the housing such that it holds the layer structure under spring tension in the housing. Furthermore, in the housing a plurality of contacts, which are electrically connected to the at least one PTC element, are mounted in an insulated manner. These contacts are normally formed as sheet metal bands and are provided extending in the longitudinal direction of the layer structure as part of the said bands. The housing is an open housing, i.e. it is normally formed with oppositely situated housing openings between which the layer structure is provided.
An auxiliary heater of this nature for conditioning the air in the interior of a motor vehicle is for example known from EP-A1-1 564 503. The layer structure comprising the radiator elements and the heat emitting elements is normally also designated as a heating block. The spring element is essentially located at the level of the heating block, i.e. between the upper and lower sides formed by the heating block, so that the spring force exerted by the spring element is essentially introduced centrally in the heating block.
The heating block of the generic electrical heating device normally comprises a plurality of parallel layers of heat emitting and heat generating elements. The heat generating elements of the heating block normally comprise several PTC heating elements which are provided in one plane one above the other, and are arranged between strip conductors which are normally formed by sheet metal bands. These strip conductors carry current with different polarities. The PTC heating elements can be glued to these strip conductors. It is also possible for the strip conductors to contact the PTC heating elements under tension. In any case it must be ensured that for extracting the heat produced by the PTC heating elements and for feeding current, good contact between the strip conductors and the PTC heating elements exists.
One or more heat generating elements can be provided as part of the heating block. The heat produced by the heat generating elements is dissipated through the heat emitting elements to the medium to be heated, i.e. the air. It flows through the housing through the two frame openings which, between them, accommodate the flat heating block. The frame openings here lie normally parallel to one another on oppositely situated sides of an essentially flat, frame-shaped housing. With regard to the most economical manufacture of the electrical heating device, the heat emitting elements are generally formed from meander-type bent sheet strips, which form corrugated ribs. These corrugated ribs contact heat emitting elements on one or both sides. Consequently, the heating block comprises several layers of heat emitting and heat generating elements, whereby it must also be ensured with regard to the thermal emission that the heat emitting elements have a good contact to the heat generating elements. Also in this respect, the heat emitting elements can be permanently joined to the heat generating elements and/or they can contact one another under tension through at least one spring element accommodated in the housing.
Instead of a meander-type sheet metal band, the heat emitting element can also be formed by an extruded aluminium profile, which forms ridges, which extend essentially at right angles to the layers of the layer structure comprising the heat emitting and the heat generating elements. In a case of this nature the strip conductor, i.e. the generally flat locating face, for the PTC heating element can be formed by the outer surface of an extruded aluminium profile of this nature. With both alternatives, corrugated rib element or extruded profile, the locating faces for the PTC heating elements are formed electrically conducting and are electrically connected to contacts which are normally mounted in the housing insulated from one another. In the first case the contacts are generally formed by the exposed ends of the sheet metal bands.
The layered heating block consisting of parallel heat emitting and heat generating elements, optionally with an additional one or more spring elements extending parallel to it, is preferably mounted in a housing with a U-shaped cross-section. When the layer structure is subjected to the pressure of a spring, the frame has to be dimensioned such that the spring force can be continuously maintained even at increased temperatures. Here it should be noted that the insulating frame is nowadays manufactured as a plastic injection moulded part, due partially to reasons of economy. Normal housings nowadays consist of a housing lower part and a housing upper part. The housing lower part here forms a receptacle for the individual elements of the heating block and, where required, of the spring element. The individual elements of the heating block are arranged in this housing lower part. Then the heating block is enclosed in the housing by joining the housing upper part and the housing lower part. To achieve this, edges which surround the frame openings can partially cover the heating block so that the heating block is enclosed between the frame openings and mounted in the housing. The two housing parts are then joined together, for example using a latching connection.
Furthermore, the housing itself should be able to be manufactured as simply as possible with regard to an economical manufacture of the electrical heating device. Here however, the particular requirements for the practical installation of one or more spring elements in the housing have to be followed when on joining the housing parts the heating block is already subjected to prestressing in the frame so that joining has to take place against this prestressing.
With EP-A1-1 432 287 a suggested solution has already been made by this applicant in which all parts forming the heating block are first inserted into the housing, the housing is then closed and thereafter a spring element, through which the layers of the heating block contact one another under the tension of this spring, is introduced from the face side of the housing through a channel. With this prior art suggested solution the layers of the heating block are first integrated into the housing without tension.
With the method known from EP-A1-1 564 503 for the production of the electrical heating device the elements forming the heating block are fitted into a housing part. Also, the spring element is fitted into the housing part. Then the housing is closed by fitting a further housing part onto the first housing part. According to the teaching of EP-A1-1 564 503, the spring element is only put under tension on closing the housing and in fact by displacement elements, which are moulded on the further housing part and act against the spring element. This displacement element forms an oblique surface on which the spring element slides past with its free upper limb. The spring element is here pivoted in the direction of the heating block about a support within the first housing part and finally tensioned to generate a tensile force.
This prior art suggestion leads to a certain simplification during assembly, which however requires that the elements of the heating block, as also the spring element, are brought into the correct positions in the housing lower part. Also, the housing implemented with this electrical heating device has various oblique surfaces, which are required for stressing and enclosing the spring element when joining the housing parts. Furthermore, the spring element must be formed in a relatively complex manner so that it fulfils its set task. A further development which essentially follows the provisions of EP-A1-1 564 503 is known from EP-A1-2 017 546. Also with this state of the art the layers of the layer structure are first introduced into the housing without being under tension. The spring element is meanwhile positioned above the plane comprising the layer structure and is displaced into the plane containing the layer structure when the housing is closed by joining the top part of the housing to the bottom part of the housing. Accordingly, on closing the housing to encase the heating block the spring element is brought into its installation position and put under tension.
As the earlier suggestions quoted above as examples show, professional circles continually endeavour to provide suggested solutions to facilitate assembly of the electrical heating device essentially without tension. In particular the elements forming the layer structure should be used initially free of tension in a housing part. The tensioning of the spring occurs in a last step in the method, i.e. after closure of the housing, preferably in a manner such that the elements of the heating device accommodated in the housing cannot be forced out of the housing by the uncoiling spring.